How trace elements are recycled in the deep sea

The oceans are full of living things, with microscopic algae (phytoplankton) at the base of the marine food chain. These organisms make a living in the same way as land plants, using the sunlight that penetrates the upper 100 meters or so of the ocean as the energy source by which they synthesise organic matter for their cells. Every year, these tiny algae make about as much organic carbon as land plants. Like land plants, they obtain the building blocks of their cells from the surrounding environment ¨C not a soil in this case, but the seawater solution they live in.

But unlike the land ecosystem, when these algae die, they fall into the dark deep ocean, where their dead cells decay due to the action of bacteria. Therefore, the elements they need to grow are lost from the part of the ocean in which they live, and go back into seawater solution in the deep ocean.

Phytoplankton removes trace elements

Somehow these elements must be returned from the deep ocean again to the surface where the whole cycle can begin again. The elements these organisms need are the same as on land ¨C carbon of course, nitrogen and phosphorous - the elements that are applied to agricultural land in fertilisers - and the many metals that all life requires, like iron, zinc and others.

Phytoplankton are important for our climate because the carbon they remove from the surface ocean is removed from contact with the atmosphere into the deep ocean, keeping atmospheric carbon dioxide lower than it would otherwise be. In the discussion of strategies to mitigate current and future CO₂ rise, one option is to massively increase the rate at which oceanic algae do all this.

But, in fact, the rate at which they do it depends on the availability in the seawater solution of ¡°nutrient¡± elements ¨C the nitrogen, phosphorous and trace metals that are very scarce in the upper sunlit ocean. So, how these elements are removed from the upper ocean and recycled back there from the deep is crucial for how the past, current and future climate of the Earth operates.

Metals integrated into manganese oxide particles

In the new paper, ETH Zurich researchers lead by geochemist Derek Vance have used tracers of ocean chemistry to discover that a substantial proportion of many of the metals are, in fact, removed quickly and permanently from the seawater solution by a process other than biology: by incorporation into solid manganese-oxide particles that precipitate from seawater and which fall all the way through the ocean into the sediment at the bottom.

But they have also discovered that the metals are returned to the deepest seawater by chemical reactions that take place in the sediment and that release the metals from the solid manganese oxide, back into solution. Finally, we have used a numerical model of the transport physics in the ocean to show that the metals released to solution within the sediment, and that leak across the interface between the sediment and the deep ocean, are mixed back up through the ocean.

Understanding climate better

¡°Our study changes how we view ocean chemistry, and its impact on ocean biology and climate¡±, Derek Vance says. For the first time, it shows that leakage of material that was once thought to be permanently lost from the oceans to the solid sediment at the bottom is crucial to how researchers think about the seawater solution and the many elements it contains that are crucial to how ocean biology works.